The main types of electrically rewritable non-volatile memories include NOR and NAND types. Compared to NOR flash memory, NAND flash memory has a slow access speed, a slow write-program speed, and a long blanking time. NAND flash memory, however, is capable of reading data in burst mode, has wide effective bandwidths, and is capable of programming and erasing a number of bits at once. Further, since write data is fetched in burst mode and a number of bits can be programmed at once, NAND flash memory has wide effective bandwidths. These features explain the popularity of NAND flash memory in various applications including memory cards and USB memories. In recent years, NAND flash memory has gained application in mobile phone memories. If NAND flash memories with hundreds of bytes became available, they could be used as an alternative to HDDs used in PCs.
A non-volatile memory as represented by NAND flash memory requires a write time as long as 200 μs and a read time as long as 50 μs. Accordingly, a page buffer capable of providing high-speed operation for temporal data storage needs to be provided. Currently available NAND flash memories lack capacitor elements of types that have a large capacitance per unit area, such as trench type and stacked type, which are necessary with the conventional DRAM. Accordingly, DRAM cell cannot be manufactured without making a significant change to the manufacturing process.
For the reasons stated above, SRAM cells are used as page buffer of NAND flash memories because of the large cell area. Accordingly, increase in capacitance of the page buffer in the interest of improving performance of the NAND flash memory will greatly increase the area of the chip. Further, usage of a DRAM cell instead of the SRAM cell as a high-speed buffer would require a significant change in manufacturing process. This results in a significant increase in manufacturing cost.